TuxGuitar can’t import that musical symbols from PTB files because of the way they are stored.
In PTB files, each track may have its own musical symbols (repeats, endings, directions, tempos, time-signatures and stuff), while in TuxGuitar, they are stored once per song and applied to every single track.
In other words, you may have a repeat applied to the guitar track while it’s not applied to the bass track. When reading that file, TuxGuitar would find that repeat and apply it using of course, TuxGuitar‘s way. If this would happen, you’d end up having that repeat applied to the guitar and bass tracks and the song would not be the same as the original one.
Ignoring this musical symbols is not good either. So, to be able to import PTB files, tuxguitar-ptb plugin, applies these symbols on the fly and remove them, trying to do its best to give you exactly the same song as the original one.